1. Field of the Invention
The present invention relates to an optical fiber preform manufacturing method and an optical fiber.
2. Description of the Background Art
Silica glass optical fibers having a core including alkali metal are known (see Patent Literatures 1-10). It is said that when an optical fiber preform is drawn, the viscosity of the core portion thereof can be reduced if the core portion contains alkali metal on the order of several hundred to several ten thousand parts per million, and accordingly the relaxation of silica glass network structure progresses, enabling decrease in the attenuation of the resulting optical fiber.
A diffusion method is known as a method of adding an alkali metal element into silica glass (see Patent Literatures 1 and 2). The diffusion method is such that material vapors of alkali metal or alkali metal salt used as materials are introduced into a glass pipe with a carrier gas while the glass pipe is heated to a temperature in the range of 1500° C. to 2200° C. by an outside heat source, and thereby the alkali metal element is diffusion-doped into the inner surface of the glass pipe.
After the alkali metal element is added in such manner to the inner surface and neighboring portion of a glass pipe, the glass pipe is subjected to diameter reduction by heating. After the diameter reduction, the inner surface of the glass pipe is etched at an appropriate thickness in order to remove a transition metal element, such as Ni, Fe and the like, which have been added simultaneously when the alkali metal is added. Since the alkali metal exhibits quicker diffusion than the transition metal element, it is possible to keep the alkali metal to remain even if the transition metal element is removed by etching the glass surface at a certain thickness.
After the etching, a core rod containing an alkali metal element is fabricated by collapsing the glass pipe by heating. And an optical fiber preform is produced by forming a cladding part on the outside of the alkali metal-containing core rod. Thus, an optical fiber can be manufactured by drawing the optical fiber preform.
In the diffusion method, vapors of alkali metal salt, which are generated by heating an alkali metal salt up to a melting point (for example, melting point of potassium bromide: 734° C.) or higher temperature so as to obtain sufficient vapor pressure, are supplied with a carrier gas to the inside of a glass pipe from an end thereof. And by heating the glass pipe by means of a heat source performing relative movement in the longitudinal direction of the glass pipe, the alkali metal element is oxidized so as to thermally diffuse into the inner side of the glass pipe. Thus, by performing such thermal diffusion process two or more times, a glass pipe containing the alkali metal element is produced.